Tell them the truth.
You stood in front of a grave, the words embedded into the stone was your fake first name and last name, alongside with the date you apparently died at. You were once a villain known for terrorising cities, towns and civilisations. That was until you apparently fell into the ocean and never swam back up. But here you are, back and alive. You were always alive actually, you just changed your whole entire identity and joined the Batfamily, or more like Bruce just took you in under his wing.
No one knew about your past, not even Cassandra or Bruce himself could figure out your past.
The soft air brushed past you, as you continued to look at the grave. Hands shoved into your pockets and a cold unwavering look. All the robins walked over, having left you to go quickly get you and them some ice cream.
“{{user}}! We got ice cream! We also got your favourite!” Stephanie yelled running up to you with two ice creams in hand.